The present invention relates to the field of material handling, in particular to a method and apparatus for preventing transfer of objects having wrong dimensions or orientation. The invention may find use in systems utilizing part handlers for gripping, transporting, and unloading objects in the hard-to-get or completely inaccessible areas, such as zones inside chemical reactors, interiors of vacuum chambers, or shielded areas for manipulations with radioactive substances.
One of the major methods for manipulating objects in special environments, such as interiors of chemical reactors, vacuum chambers, or shielded zones for manipulating radioactive substances, is the use of part handlers or grippers for grasping, holding, moving, and positioning of objects. In some cases, a part handler does not manipulate the parts directly but rather handle intermediate pallets, holders, or plates, which support the parts to be processed. For example, a part to be treated inside a chemical reactor can be transported to a temporary storage to free the reactor for another operation. Upon completion of this operation, the part handler returns the pallet or a holder to the chemical reactor for further processing.
For example, U.S. Pat. No. 6,209,220 issued in 2001 to Ivo Raaijmakers discloses a device for cooling substrates in a semiconductor processing system, e.g., in a cluster tool. According to one embodiment, a separate plate of a material with good heat-conductive properties is kept cool within a pocket of the cluster tool chamber during processing and is moved closer to the substrate for use as a heat sink. The cooling plate is moved into and out of the aforementioned pocket in a lateral direction, or the wafer support is raised towards the cooling plate in a vertical direction.
In the case when a part or a part-supporting holder, such as an elliptical, rectangular, or circular plate, participates in a technological process and has to be transferred from one position to another by a part handler or another part handler, it should be provided with means for accurate placement into the initial position from which is has been transferred to the temporary storage. In the case the alignment is not achieved and the part or a holder is gripped and returned to a wrong position, this may lead to crash of the part or lead to serious damage of the equipment.
One alignment or centerfinding system is described in U.S. Pat. No. 5,483,138 issued to S. Shmookler, et al., in 1996. The aforementioned system uses four photoelectric position sensors to locate the center of a wafer as the wafer is moved between chemical reactors. An array of optical sources is disposed above the central wafer transport chamber and a corresponding array of optical detectors is disposed below the chamber. The illustrated sensor array allows the identification of wafer positions, but requires that the wafer transport chamber be optically accessible from both the top and the bottom of the chamber. This photoelectric sensor array is arranged so that the light travels along a path generally perpendicular to the plane in which the wafer is transported. In practice, this type of sensor geometry may lead to erroneous position information due to multiple reflections from the surfaces of the top and bottom chamber covers and from the wafer. To compensate for such erroneous position data, the Shmookler system samples more data points than necessary, discarding data that does not fall within expected limits.
The Shmookler system uses a data collection scheme, which relies on particular points on the edge of a wafer crossing the four-sensor array in a particular order. This scheme works well when wafers are positioned near to their nominal position. However, wafers that are in danger of breaking in the course of a transport operation may be dislodged from their nominal position by a large amount. For such substantially misaligned wafers, the Shmookler system will not appropriately identify the wafer position, and wafers that are substantially out of position may consequently be broken.
U.S. Pat. No. 5,740,062 issued in 1998 to Berken, et al., which is aimed at elimination of disadvantages inherent in the Shmookler system, discloses a wafer positioning system which determines the position of a wafer during processing by monitoring the position of the wafer transport robot as the robot transports the wafer by one or more position sensors. The wafer positioning system incorporates a transparent cover on the surface of the wafer handling chamber and two optical position sensors disposed on the surface of the transparent cover. The position sensors direct light through the wafer-handling chamber to reflectors near the floor of the chamber, which reflect the light back to the position sensors. A detector within the position sensor detects when the beam path from the position sensor to the reflector is uninterrupted. As wafers are transported through the chamber, the edge of the transported wafer interrupts the position sensor beam path causing the output of the position sensor to switch states. When the position sensor output switches, the position of the wafer transport robot is measured. At least two data points are measured to establish the wafer position. If the wafer is not at its nominal position, the position of the wafer transport robot is adjusted to compensate for the wafer misalignment.
A main disadvantage of all known systems described above consists in that they cannot operate with parts subject to change in dimensions. For example, after multiple participations in sputtering or chemical processes, a carrier or part handler may change the outer diameter due to deposition of sputtered materials or due to etching. If the carrier is aligned with the use of positioning pins and openings and the position is controlled by sensing the edge of the object having a given diameter for finding the center of the object, such a method may lead to miscalculations in centerfinding. This is because the diameter is changed. As a result, the positioning pins will not be aligned with the openings, so that the handler will not return to its correct initial position, or the part supported by the carrier will be picked up in a wrong position. As mentioned above, this may lead to serious problems.
It is an object of the invention to provide an apparatus for preventing handling of objects having wrong dimensions or orientation. Another object is to provide an apparatus for automatically stopping an operation of gripping of misaligned parts with a parts handler. Still another object is to provide an apparatus capable of centering objects with variable dimensions. A further object is to provide an apparatus of the aforementioned type which is simple in construction, inexpensive to manufacture, does not involve calculations for determining the central position of the object, and can be easily built into an existing system. Still another object is to provide a method for preventing gripping of objects having wrong dimensions or orientation.
The invention describes an apparatus for preventing gripping of objects having wrong dimensions or orientation. The apparatus comprises a part handler, e.g., a holder for parts to be treated in a chemical reactor, where the parts has to be transferred from a working position to a temporary storage. The holder may have different shapes, e.g., rectangular, elliptical, or circular, and is provided with positioning openings or recesses for engagement with pins or semispherical elements on the engaging surface of the part handler. The apparatus is provided with at least two through beam optical sensor units with adjustable divergence of the light beams emitted from the light emitting to the light-receiving element. The sensor units are located near the edge area of the holder. Position of the holder for aligning with the part handler is determined by combined interaction of the holder edge with the sensor units so that, depending on overlapping of one or two sensors and on the position of the overlapped zone, the apparatus determines whether or not the holder is aligned with the part handler or the gripping operation has to be rejected. In another embodiment the apparatus is equipped with three optical sensors, which simplify the control operation.